TECHNIQUE E: VOLUMETRIC TRANSFER OF REAGENT USING A PIPET
SECTION 1: PURPOSE OF TECHNIQUE
This technique describes how to use a volumetric pipet to collect, measure, and transfer a specific, precise volume of liquid. “Pipet” and “Pipette” are both correct spellings and “pipet” will be used here.
SECTION 2: OPERATIONS FOR A VOLUMETRIC PIPET TRANSFER
A common type of volumetric glassware is the volumetric pipet. It is an elongated glass tube with two, slim hollow glass tubes on either side of a glass bulb. The top end of the pipet is a fully-open tube, the bottom end of the pipet is tapered to a fine point. When held upright, above the glass bulb there is a calibration line that specifies the volume of liquid withdrawn at a specific temperature. The tapered tip that allows a precise stream of liquid to be dispensed from the bottom of the pipet. The volumes are fixed and marked on each pipet. The sizes often found in chemistry labs are 1.00-ml (milliliter), 2.00-ml, 3.00-ml, 5.00-ml, 10.00-ml, and 25.00-ml pipets. A rubber pipet bulb is used to generate suction and control the amount of reagent collected by the volumetric pipet.
- Obtain two clean and dry beakers or other containers. One will be used for stock reagent. One will be used to receive the dispensed liquid.
- Collect enough reagent for the procedure. Obtain a volumetric pipet of the appropriate volume for the experiment and a rubber pipet bulb.
- Start with a dry pipet, or rinse the pipet with a little of the solution that you are measuring. If rinsing, try to coat all interior surfaces with the solution to be dispensed. Discard the rinse.
- Several styles of pipet bulbs are available. The style shown does not affix to the top of the pipet, it is pressed briefly at the end of the pipet to generate a temporary seal to apply suction.
- Hold the pipet vertically from the top. Hold the pipet bulb with the other hand and expel air from the bulb. Submerge the pipet tip below the surface of the reagent. Use the pipet bulb to provide suction and fill above the calibration line of the pipet.
- Stop suctioning when the meniscus is a few centimeters above the calibration mark. Keep the pipet tip submerged in the solution. Be careful as the height of the liquid being sucked into the pipet gets faster as you get to the top.
- When the liquid is above the calibration mark, remove the pipet bulb and quickly cover the opening of the pipet with a finger. Align your eye to the calibration mark so that is appears as a single paint line. Gently release the seal with your finger to allow the solution to drain from the pipet due to gravity. Let the meniscus move down until it is just touching the paint line of the calibration mark.
- Meniscus is right on the calibration line.
- Meniscus is not aligned with the calibration line.
- Withdraw the pipet from the solution and quickly wipe the exterior of the pipet to make sure there are no liquid droplets on the exterior of the pipet. Place the pipet over the receiving container and allow the liquid to drain. Once the pipet has fully drained, wait an addition 15 seconds for any interior flow to complete.
- Tap the surface of the liquid three times with the tip of the pipet to reduce any surface tension built up in the beaker. Do not blow out the last drop from the pipet. The transfer pipet is calibrated to contain this last drop.
SECTION 3: TIPS FOR A PRECISE MEASUREMENT
A precise measurement of volume will not occur if the volumetric pipet is mishandled. When handling a volumetric pipet, there are certain factors that should be evaluated before throughout the procedure:
- The tip of the pipet should be examined before the experiment begins to check if there is any damage that can affect the accuracy of the measurement. If the tip is chipped or shows any sign of damage that contacts the center hole of the pipet tip, insufficient surface tension will be present to maintain a constant size of the remaining drop in the transfer pipet. The pipet is likely not repairable and should be discarded.
- It is important to hold the pipet from the top stem rather than in the middle. The middle of the pipet is made of very thin glass and can easily be broken when lateral force is applied.
- Before placing the pipet bulb onto the opening of the pipet, squeeze the bulb to expel air before contacting it on top of the pipet. This will seal any air around the pipet that will make it hard to collect any liquid. It is not necessary to squeeze the bulb so vigorously to capture liquid. Slow presses on the bulb will allow enough liquid to come up and will give you control on the speed the liquid is collected at.
- If the meniscus is not a little higher than the calibration mark while the pipet is submerged into the liquid, then the volume collected will not result in a precise mass. Once the pipet is taken out of the liquid, the change in volume allows the liquid to decrease which reduces the amount of mass the balance will read.
- When transferring liquid using the volumetric pipet, it is important to use an index finger to stop the flow of liquid rather than the thumb. Thumbs tend to have larger areas of contact, which lead to less control of releasing pressure.
- While the reagent is streaming down, do not let the tip of the pipet hit the liquid’s surface or the sides of the beaker.
SECTION 4: FURTHER INFORMATION
Getting accurate results using the volumetric pipet takes practice. In order to achieve precise measurements of volume for an experiment, you must know how the pipet works and how to take care of it properly. For further information on using a volumetric pipet, check out the resources provided below.